High Consistency Re-Pulping Method, Apparatus and Absorbent Products Incorporating Recycled Fiber

ABSTRACT

A method of pulping wastepaper includes providing a pulping vessel with a rotor and at least one flow sensor adapted to measure slurry flow within the pulping vessel. The pulping vessel is charged with wastepaper and water to form a slurry, the amounts of wastepaper and water being present such that the slurry has consistency in the range of from 10% to 30%. The wastepaper charge in the pulping vessel is pulped at a pre-selected power level while monitoring poloidal slurry flow in the pulping vessel; and from time-to-time, water may be added to the pulping vessel when the poloidal flow falls below a predetermined lower threshold flow value in order to reduce viscosity and thereby restore poloidal flow within the pulping vessel. Doppler velocimetry is a preferred method of monitoring pulp flow. In a particularly preferred construction the rotor has a variable power drive. The inventive method is especially useful for making high quality, high brightness furnishes from wastepaper which varies from batch to batch.

CLAIM FOR PRIORITY

This application is based on U.S. Provisional Application No.62/512,800, filed May 31, 2017 of the same title, the priority of whichis hereby claimed and the disclosure of which is incorporated herein byreference.

TECHNICAL FIELD

The present invention relates to wastepaper recycling for papermanufacture utilizing high consistency pulpers and absorbent productsincorporating recovered fiber.

BACKGROUND

Wastepaper pulping is the first step in producing a recycle papermakingfurnish. The main measurements of the effectiveness of pulping are: 1)yield, 2) Effective Residual Ink Concentration (ERIC)—an optical testwhich measures the presence of fine ink particles at 950 nm in astandard handsheet or filter pad of fiber, 3) handsheet brightness, 4)stickies content, 5) ash content, 6) ink (“dirt”) specks or particles,and 7) Visual Speck Index (VSI). VSI uses a dilute sample of pulp whichis visually compared to a standard reference to measure the percentageof the sample that is pulped (absence of fiber flakes).

While virgin fiber tends to be of higher quality than recycle fiber, itcosts more than recycle fiber and many environmentally consciousconsumers actually prefer absorbent products such as paper towel andpaper tissue made with recycled fiber.

Wastepaper recycling typically includes, in addition to re-pulping,de-inking and bleaching the wastepaper. U.S. Pat. No. 5,785,809,entitled “Biological De-Inking Method” discloses de-inking wastepaperwith enzymes. U.S. Pat. No. 5,234,545, entitled “Process forRegenerating Wastepaper in the Presence of Hydroxysulfonate Salts”provides typical conditions for wastepaper recycling in Col. 4, lines24-42. U.S. Pat. No. 4,865,690, entitled “Process and Plant forRecycling Waste Printed Papers” teaches a recycling process forwastepaper including re-pulping, de-inking and bleaching. Wastepaperrecycling places demands on the equipment, especially because a pulpslurry flow is abrasive over time and because of the presence of foreignobjects such as glass, wood or metal fragments, plastics and other trashin the recycle stream. U.S. Pat. No. 7,013,287, entitled “StockPreparation Monitoring System and Method of Same” discloses ultrasonicsensing of damage or wear to screens, augers, foils, rolls and so forthin a paper stock processing system.

A variety of re-pulping methods are commercially practiced, the threedominant technologies are 1) drum continuous pulping, 2) low to midconsistency pulping (batch and continuous), and 3) high consistencybatch pulping. The trend in the industry has been towards low energydrum pulping and reduced energy batch pulping systems. The trend islargely based on the conventional wisdom that the pulping technologyutilized is not a critical factor in the ultimate pulp quality becausemodern deinking systems have the capability to compensate for anydeficiencies in pulping. Contrary to conventional wisdom, it has beenfound in accordance with the present invention that high consistencypulping of wastepaper is preferred, provided that the pulping process isadequately controlled to avoid over-pulping the wastepaper.

High consistency wastepaper re-pulping is described in Hong, Y. et al,“Repulping Characteristics of Wastepaper at High Consistency withHelical Repulper”, from Research Polymers in Pulping and Papermaking[International Symposium on Emerging Technologies of Pulping andPapermaking], 3^(rd), Guangzhou, China, Nov. 8-10, 2006, pp. 854-859which discusses re-pulping of wastepaper in a laboratory size highconsistency pulper.

The following references also relate to high consistency pulping. U.S.Pat. No. 6,988,682, entitled “Method and System for Utilizing Waste”discloses recycling with a continuous operation high consistency pulper.See Abstract. U.S. Pat. No. 6,651,679, entitled “Process for theBreaking and Cleaning of Contaminant-Containing Raw Paper Stocks, inParticular of Recovered Paper” teaches a recycling process which mayinclude a high consistency pulper. See Col. 3, lines 44-49. U.S. Pat.No. 5,958,179, entitled “Process for Increasing Pulp Whiteness byBleaching Printed Wastepaper Under Intense Dispersing MechanicalTreatment” relates to high consistency recycling of wastepaper as doesU.S. Pat. No. 5,538,594, entitled “Method for Producing a Blade CoatedPaper from Recycled, High Lignin Content, Waste Paper”. U.S. Pat. No.5,520,783, entitled “Apparatus for Bleaching High Consistency Pulp WithOzone” discloses high consistency bleaching of pulp with ozone. See alsoU.S. Pat. No. 4,593,861, entitled “Apparatus for Pulping Paper MakingStock at High Consistencies”.

U.S. Pat. No. 4,535,943, entitled “Pulping Apparatus Including a Rotorand Helical Screw Flights Extending Upwardly From the Rotor” discloses abatch pulper. Note FIG. 1 of the '943 patent. This reference disclosespulping at consistencies of 12% to 25% solids followed by adding waternear the end of processing to flush the re-pulping vessel. See Col. 6,lines 34-46.

Conventional practice in connection with high consistency pulping is torun a recipe and a timed sequence of operations, typically including afixed amount of wastepaper, a fixed amount of water and a predeterminedpulping interval. Conventional high consistency pulping can lead toresults which are highly variable in terms of brightness (with thepresence of under-pulped fiber flakes also sometimes being a significantproblem) reducing yield and quality. It is believed that brightnessvariability is somewhat inherent to wastepaper differences frombatch-to-batch and that no method exists to predict the optimal pulperrecipe prior to actual pulping. The viscosity of a wastepaper slurry isimpacted by many factors including the feed wastepaper moisture level,ash and coating levels, and the types and morphology of the papermakingfibers utilized to manufacture the original paper products. It shouldalso be noted that the cycle time for high consistency pulping is veryshort, as low as 8-12 minutes at full rotor power, so that anyadjustments, if any, must be done very quickly. While it might bepossible to test each bale of wastepaper for processing characteristics,such testing would be cost and time prohibitive.

High consistency pulping has advantages over other methods in terms ofcapital costs, yield, ink removal, breakdown of flakes, fiberthroughputs, ease of operation, wastewater volume and operating expense.However, it is seen that high consistency pulping as conventionallypracticed often has an adverse effect on the brightness of the finishedpulp because there are relatively high levels of residual ink in theproduct which persist throughout further processing. Without intendingto be bound by any particular theory, it is believed that high energypulping taken to very low flake levels results in irrecoverablebrightness loss because the fine inks reattach to the fibers fromfrictional forces. These inks are not easily removed in deinkingsystems. The key to preferred processing in accordance with theinvention is to reliably apply the correct amount of energy to achievethe optimal point where you get a high yield, good ink breakup (lowdirt), high throughput and no loss in brightness.

SUMMARY OF INVENTION

There is provided in one aspect of the invention a method of pulpingwastepaper comprising: (a) providing a pulping vessel with a rotorhaving a power drive and at least one flow sensor adapted to measureslurry flow within the pulping vessel; (b) charging the pulping vesselwith wastepaper and water to form a wastepaper slurry, the amounts ofwastepaper and water being present such that the slurry has consistencyin the range of from 10% to 30%; (c) pulping the wastepaper slurry inthe pulping vessel while monitoring poloidal slurry flow in the pulpingvessel; and (d) maintaining poloidal slurry flow in the pulper at orabove a lower threshold slurry flow value during pulping.

It has been discovered that the pulping method employed in wastepaperrecycling has a persistent impact on the finished stock qualityregardless of the deinking system. The present invention provides amethod to produce high value pulps from wastepaper by way of a uniquecontrol strategy for high consistency pulping. The method mostpreferably involves directly controlling the rate of energy input andthe total specific energy used in pulping through a variable power rotorand through maintaining a target poloidal velocity in the repulpingvessel. Velocity control of the pulp slurry is conveniently achieved bymonitoring the poloidal velocity of the material in the pulper whileadjusting viscosity of the charge by adding water from time to time asnecessary in order to maintain poloidal flow in the pulper above a lowerflow threshold. Poloidal velocity in the pulper is most preferablymonitored with Doppler velocimetry. Without intending to be bound by anytheory, it is believed that the inventive method avoids “over-pulping”wherein ink is smeared or reattached to the fiber being processed.Over-pulping is a function of both pulping time and power application.Consider what happens in the pulper during the high consistencyprocessing cycle. At the beginning, the pulper only contains the watercharge. As the wastepaper is added there is a very fluid mixture ofwater and fairly intact wastepaper and wastepaper flakes. This mixturehas a viscosity close to water and circulates fairly rapidly. The shapeof the mass is an elongated toroid and the flow is both toroidal(circular around the major axis) and poloidal (circular around the minoraxis i.e. flow from the bottom of the tub, up the wall of the vessel,back towards the center and down through the rotor, see arrow 34, FIG.1). Poloidal flow is thus conveniently characterized by upward velocityof the slurry near the wall of the pulper in most cases. A directionalDoppler effect ultrasonic flow meter is most preferred. These devicesmeasure pulp slurry velocity in the direction indicated on the meter andshould be positioned on the pulper in areas of low turbulance, such asaway from the base of the rotor for best results. As the individualfibers are liberated the viscosity of the pulp mixture increases and theflow slows down. The increased viscosity also increases the power drawof the motor. As pulping continues the ash and coatings in the fiber areliberated and broken up further increasing the viscosity. After a fewminutes the toroidal flow essentially stops and most of the flow ispoloidal. The power application reaches its maximum and the rotor tofiber and fiber to fiber forces continue to break up the inks and fiberflakes. At the end of the cycle dilution water is added and the pulpmass viscosity drops and both toroidal and poloidal flows of relativelyhigh velocity are seen.

In many of the wastepaper pulping cycles observed while developing theinvention, discussed hereinafter, it is believed the viscosity increasedto the point where significant poloidal flow also stopped during apulping cycle. The pulp mass near the walls of the pulper and at the topof the pulper stop circulating and only the pulp in the vicinity of therotor was moving. The consequences of the drop in flow include: 1) muchof the pulp is under-pulped as it only circulates for the first part ofthe pulping cycle, and 2) the pulp at the bottom and near the rotor isover-pulped causing ink smearing and excessive comminution of dirtparticles making them difficult to separate in a de-inking system andadversely impacting brightness of the finished pulp.

The present invention provides a method and apparatus to practice highconsistency pulping while avoiding over-pulping of the wastepapercharge. A salient feature of the invention is to maintain poloidal flowin the pulper above a lower threshold as is seen in FIG. 2, for Data Set5. FIG. 2 shows a steadily increasing energy application with sufficientflow in all regions of the pulper throughout the re-pulping cycle.

The most preferred results are achieved through selection and control ofpulping time, power application and the viscosity of the charge to thehigh consistency pulper; these latter two features being related for agiven system as is noted below.

The power draw of the pulper motor is a complicated function ofvariables including the type of motor, the speed of the motor, the gearratio of the gear box connecting the motor to the rotor, the rotordesign, the pulper tub design, the amount of water in the pulper, theamount of wastepaper in the pulper, the characteristics of thewastepaper and how broken up or individualized the wastepaper is at agiven time in the cycle. For a given system design and set of the abovevariables the energy application to the wastepaper is directlyproportional to the viscosity of the pulp mass and the speed of therotor up to the point of excessive rotor slip or cavitation. At idealconditions there is a smooth flow of pulp into the top of the rotor atthe center of the pulper. The rotor accelerates the pulp as it travelsdownward and outward and the pulp is flung out from the bottom of therotor with sufficient force to cause a poloidal flow across the bottom,up the sides and back to the top center of the pulper where it reentersthe rotor. As the rotor speed is increased the resistance to flowincreases and the load on the rotor will increase.

As the rotor speed is increased the torroid shape of the pulp mass alsobegins to elongate in the direction of its vertical axis as the upwardforce along the pulper wall starts to become larger than the constantgravitational force which pulls the pulp downward and inward towards thetop of the rotor. As rotor speed increases, at some point the rotorbecomes starved as the inflow of pulp to the center of the pulpingvessel can no longer keep up with the outflow along the bottom of thepulping vessel to the vessel wall and the rotor power drops. The pulpalso begins to short circuit the pulper wall so that the pulp nearer thebottom will continuously circulate through the rotor while the pulphigher will stagnate and eventually stop circulating.

The second critical factor in operating the high consistency pulper isthe viscosity of the pulp mass. At the initial addition of wastepaper tothe water in the pulper the viscosity of the fluid is essentially thatof water. As the mass of paper begins to breakdown into smaller piecesand individual fibers the viscosity increases substantially. Thewastepaper can also include additives such as clay, starch, calciumcarbonate, titanium dioxide and others. These additives are releasedfrom the paper and increase the viscosity. As the fillers and individualfibers increase the viscosity the resistance to flow increases and thecritical rotor speed is reduced where starving and cavitation occurs.Once the rotor starts to cavitate and the pulper short circuit theresult is some fraction of the fiber is over pulped (the shortcircuitingfiber) and some is under pulped (the upper mass of fiber that stopsmoving). Depending on the situation the average energy application maybe unchanged so that monitoring the power alone may not detect shortcircuiting of the pulp flow.

A preferred method of pulping is to pulp at maximum rotor power for theshortest possible time so that the pulping occurs very quickly and thebrightness loss is minimized. This requires the viscosity be very highand maintained just below the level where pulp slurry flow drops belowthe minimum value required to fully load the pulper rotor. The inventionmethod provides a way to operate the pulper in this narrow window bycontrolling the specific power application and the linear velocity inthe pulper utilizing a variable power (and speed) rotor and usingdilution water addition to adjust viscosity in response to pulp slurryflow measurements. Operating the pulper motor near its practical maximumis preferred, generally at greater than 90% of the maximum power of themotor, preferably at greater than 95% of the maximum power of the motor,such as between 95% and 100% of the maximum power of the motor. Apreferred system for powering the pulper rotor is an AC motor providedwith an AC drive. AC drives change the speed of an AC motor by changingvoltage and frequency of the power supplied to the motor. Typically theAC drive consists of rectifier and inverter units and a microprocessorto control output. In order to maintain proper power factor and reduceexcessive heating of the motor, the name plate volts/hertz ratio shouldbe maintained.

Further features and advantages will become apparent from the discussionwhich follows.

BRIEF DESCRIPTION OF DRAWINGS

The invention is described in detail below with reference to thedrawings wherein:

FIG. 1 is a schematic diagram of an apparatus for re-pulping wastepaperin accordance with the present invention;

FIG. 2 is a plot of rotor power (left ordinate) and pulp slurry poloidalvelocity (right ordinate) versus time for a pulping cyclic executed inaccordance with the present invention;

FIGS. 3A and 3B are plots of contamination versus pulping time forwastepaper;

FIGS. 4A and 4B are plots of residual ink and handsheet brightnessversus pulping time for wastepaper;

FIG. 5 is a histogram showing ink contamination for drum pulping andhigh consistency batch pulping;

FIG. 6 is a histogram showing brightness for drum pulping and highconsistency pulping;

FIGS. 7-11 are plots of rotor power versus time for various highconsistency pulping trials;

FIGS. 12 and 13 are plots of rotor power (left ordinate) and pulp slurrypoloidal velocity (right ordinate) versus time for high consistencypulping trials;

FIG. 14 is a plot of poloidal velocity, consistency, power/rpm, powerand motor speed versus time for a high consistency wastepaper pulpingtrial;

FIG. 15 is a plot of poloidal velocity, consistency, power/rpm, powerand motor speed versus time for another high consistency wastepaperpulping trial;

FIG. 16 is a plot of poloidal velocity, consistency, power/rpm, powerand motor speed versus time for yet another high consistency wastepaperpulping trial;

FIG. 17 is a plot of poloidal velocity versus time for the three (3)trials of FIGS. 14-16;

FIG. 18 is a plot of poloidal velocity and consistency for the three (3)trials of FIG. 14-16;

FIG. 19 is a plot of poloidal velocity versus motor RPM for Data Set 7,with increasing pulping time indicated on the graph; and

FIG. 20 is a block diagram of a multi-stage wastepaper pulping process.

DETAILED DESCRIPTION

The invention is described in detail below in connection with theFigures for purposes of illustration only. The invention is defined inthe appended claims. Terminology used herein is given its ordinarymeaning consistent with the exemplary definitions set forth immediatelybelow; g refers to grams, m² refers to square meters, MT means metricton, kW refers to kilowatt, percents, ppm and like terminology relatesto weight percent, parts per million by weight unless otherwiseindicated and so forth.

“Consisting essentially of” and like terminology refers to the recitedcomponents and excludes other ingredients which would substantiallychange the basic and novel characteristics of the composition, articleor process. Unless otherwise indicated or readily apparent, acomposition or article consists essentially of the recited or listedcomponents when the composition or article includes 90% or more byweight of the recited or listed components. That is, the terminologyexcludes more than 10% unrecited components.

The term “cellulosic”, “cellulosic sheet” and the like are meant toinclude any product incorporating papermaking fiber having cellulose asa major constituent. “Papermaking fibers” include virgin pulps orrecycle (secondary) cellulosic fibers or fiber mixes comprisingcellulosic fibers. Fibers suitable for making the webs of this inventioninclude: nonwood fibers, such as cotton fibers or cotton derivatives,abaca, kenaf, sabai grass, flax, esparto grass, straw, jute hemp,bagasse, milkweed floss fibers, and pineapple leaf fibers; and woodfibers such as those obtained from deciduous and coniferous trees,including softwood fibers, such as northern and southern softwood Kraftfibers; hardwood fibers, such as eucalyptus, maple, birch, aspen, or thelike. Papermaking fibers used in connection with the invention aretypically naturally occurring pulp-derived fibers (as opposed toreconstituted fibers such as lyocell or rayon) which are liberated fromtheir source material by any one of a number of pulping processesfamiliar to one experienced in the art including sulfate, sulfite,polysulfide, soda pulping, etc. Papermaking fibers may comprise a blendof conventional fibers (whether derived from virgin pulp or recyclesources) and high coarseness lignin-rich tubular fibers, such asbleached chemical thermomechanical pulp (BCTMP). Pulp-derived fibersthus also include high yield fibers such as BCTMP as well asthermomechanical pulp (TMP), chemithermomechanical pulp (CTMP) andalkaline peroxide mechanical pulp (APMP). “Furnishes” and liketerminology refers to aqueous compositions including papermaking fibers,optionally wet strength resins, debonders and the like for making paperproducts.

Kraft softwood fiber is low yield fiber made by the well known Kraft(sulfate) pulping process from coniferous material and includes northernand southern softwood Kraft fiber, Douglas fir Kraft fiber and so forth.Kraft softwood fibers generally have a lignin content of less than 5percent by weight, a length weighted average fiber length of greaterthan 2 mm, as well as an arithmetic average fiber length of greater than0.6 mm.

Kraft hardwood fiber is made by the Kraft process from hardwood sources,i.e., eucalyptus and also has generally a lignin content of less than 5percent by weight. Kraft hardwood fibers are shorter than softwoodfibers, typically having a length weighted average fiber length of lessthan 1 mm and an arithmetic average length of less than 0.5 mm or lessthan 0.4 mm.

Consistency refers to percent solids of a nascent web or slurry, forexample, calculated on a dry basis. A slurry having 80 percent water and20 percent dry wastepaper has a consistency of 20 percent.

“Air dry” or simply “dry” means including residual moisture, byconvention up to about 10 percent moisture for pulp and up to about 6percent for paper; while oven dry refers to pulp or paper which is driedin an oven for several hours and is significantly drier.

Products produced in accordance with the invention are made with acellulosic fiber basesheet and have an absorbency or SAT value as wellas tensiles and densities suitable for tissue and towel products.Typical SAT values are greater than about 3 g/g in most cases. See U.S.Pat. No. 8,778,138.

“CWP” refers to a process for making absorbent products by way of aconventional wet-press process; that is, wet-pressing a furnish to adrying cylinder with a papermaking felt followed by creping the web fromthe cylinder. See U.S. Pat. No. 7,951,266, FIG. 7 thereof.

A “Structured Basesheet Process” refers to a process for making anabsorbent product by wet creping (fabric creping) from a cylinder priorto final drying. See U.S. Pat. Nos. 7,850,823; 7,585,388; 7,585,389; and7,662,257.

A “TAD Process” refers to through-air dried processes for makingabsorbent products. Throughdried, creped products are disclosed in thefollowing patents: U.S. Pat. No. 3,994,771 to Morgan, Jr. et al.; U.S.Pat. No. 4,102,737 to Morton; and U.S. Pat. No. 4,529,480 to Trokhan.The processes described in these patents comprise, very generally,forming a web on a foraminous support, thermally pre-drying the web,applying the web to a Yankee dryer with a nip defined, in part, by animpression fabric, and creping the product from the Yankee dryer.

A towel product is typically characterized by having predominantly (morethan 50% by weight based on fiber content) softwood fiber.

A tissue product is typically characterized by having predominantly(more than 50% by weight based on fiber content) hardwood fiber.

“Wastepaper” is any paper including papermaking fibers that may bere-pulped and recycled. Wastepaper thus includes mill broke,pre-consumer waste, and post-consumer waste including office wastepaper.Mill broke is paper trimmings and other paper scrap from the manufactureof paper, and is recycled internally in a paper mill. Pre-consumer wasteis material which left the paper mill but was discarded before it wasready for consumer use. Post-consumer waste is material discarded afterconsumer use, such as old corrugated containers, old magazines, andnewspapers, while office wastepaper is a subcategory of post-consumerwaste which includes predominantly laser and xerographic printed paper.Paper suitable for recycling is sometimes referred to simply as “scrappaper”.

Recycled fiber is fiber recovered from wastepaper. While any suitablerecycle fiber may be used in producing absorbent products of theinvention, recycle fiber with relatively low levels of ground wood ispreferred in many cases, for example recycle fiber with less than 15% byweight lignin content, or less than 10% by weight lignin content may bepreferred depending on the furnish mixture employed and the application.Recycle fiber is in many cases 80% hardwood fiber.

A variable power rotor refers to a rotor coupled to a drive motor whichhas controls for setting the power level, typically as a percent of amaximum value for the motor. The rotor operates at variable speed toload the motor to the set power value, and the speed will vary dependingupon the viscosity of the charge being pulped. Such equipment iscommercially available, such as, for Example a Siemens SINAMICS GM150variable speed drive (Siemens Aktiengesellschaft, Munich, Bavaria).Preferred variable power rotors are coupled to an AC motor provided withan AC drive.

Pulp Flow, Poloidal Flow Instability, Pulping Duration and OperatingTarget Levels

Any suitable non-invasive method for measuring poloidal velocity of thepulp slurry within the pulper may be employed. Non-invasive flowmeasurement techniques based on ultrasonic methods are preferred,although techniques based on radar, Doppler radar, a 3D CAT scan orother penetrating radiation meter and the like may likewise be employed.Any method that can measure the flow without being exposed to the movingpulp mass is suitable. There are two types of ultrasonic flowmeasurement techniques, transit time and Doppler; the latter technologyalso being sometimes referred to as Doppler velocimetry. Given themotion of the pulp beneath the surface cannot be measured byobservation, or by real time rotor power data, a particularly effectiveoption is to utilize a Doppler effect ultrasonic flow meter. Thistechnique requires hardly any modification to a batch pulper other thanattaching a Doppler velocimetry meter to the tub wall. It should benoted the interior of the pulper is an extremely hostile environment fora flow measuring device. The pulp mass generates high levels of frictionand the wastepaper often contains solid metal, glass and woodcontaminants which damage any type of probe installed inside the pulpertub. A clear viewing port could be added to the wall of the pulper inorder to use an optical method; however the port would likely be quicklydamaged. Initial testing was completed using a portable Doppler meter(Cole-Parmer Model 32986-00) and it was found that Doppler velocimetryworked surprisingly well. An advantage of this method is that it isdirectional which allows both the speed and direction of flow to bemeasured in the flow direction indicated on the meter. Suitable metersfor mounting on a batch pulper tub wall include Dynasonics 5XPN1 (BadgerMeter, Milwaukee, Wis.), Dwyer Instruments 3PCL1 (Dwyer Instruments,Michigan City, Ind.) and equivalents from other manufacturers.

Poloidal slurry flow exhibits instability when the poloidal velocityexhibits a sharp drop over a relatively short time interval, typicallyfollowed by additional abrupt changes in poloidal velocity after theonset of instability of the poloidal slurry flow. The onset ofinstability is the point at which an initial sharp drop in poloidalvelocity is observed. A sharp drop in poloidal velocity is typicallycharacterized by a poloidal velocity decrease of 25% or more over aperiod of 0.3 minutes or so, a deceleration of 25%/0.3 minutes or83.3%/minute. Onset of instability in poloidal slurry flow is thustypically characterized by a deceleration in poloidal velocity of morethan 80% per minute, as compared to poloidal velocity just prior toonset of instability. Decelerations of more than 100% per minute, 150%per minute or larger are commonly observed at the onset of instability,as compared with the poloidal velocity just prior to the onset as isseen particularly in FIGS. 14-19, discussed hereinafter. In practicaloperation, it may be desirable to avoid pulp slurry flow instabilityaltogether and/or discontinue pulping shortly after pulp flowinstability occurs.

A particular level or range of poloidal velocity (or other parameter) isdeemed maintained during a pulping operation when that level or range ofpoloidal velocity (or other parameter) is preserved over a substantialportion of a pulping time interval, more than 40% of pulping duration.Pulping duration and like terminology such as a pulping time interval isthe residence time of the wastepaper in the pulper under pulpingconditions. For batch pulping described herein, the time interval forpulping starts when the pulper is charged and the motor is set topulping speed and continues until the post-pulping dilution water isadded. Post-pulping dilution water is typically provided in amountswhich significantly lower charge consistency; more than 5%, i.e., from20% consistency to less than 15% consistency. Post-pulping additionwater addition is accompanied by a sharp increase in poloidal slurryflow as is seen in FIGS. 14-18. A particular level of a parameter, forexample, poloidal velocity or range is preferably maintained orpreserved over at least 50% of the pulping time interval or pulpingduration; suitably, the particular level or range of poloidal velocityis maintained over at least 60%, 70%, 75%, 80%, 85% or more up to 90% or95% or up to 100% of the pulping time interval. “During pulping” meanswithin or during the pulping time interval.

Target level(s) and operating level(s) refer to a particular targetvalue or a range within a particular parameter is maintained. Forexample, the target level for poloidal flow may be a particular velocitysuch as 0.15 ft/sec or a range of from 0.1 ft/sec to 0.4 ft/sec.

When referring to a range, “within” includes the endpoints of the range.

Preliminary Trials

Samples of wastepaper were supplied to a pilot plant and a series ofexperiments run with pulping technology as a variable. Drum pulping wasdirectly compared to high consistency pulping with both pulps runthrough the same pilot scale deinking and bleaching process. Thedeinking and bleaching process was chosen to simulate state of the artcommercially available technology FIGS. 3A and 3B show the impact ofpulping technology on visible ink speck concentration and count. On FIG.3B nb/m² refers to the number of specks per square meter. FIGS. 3A and3B clearly show the superior ink breakup of the high consistencytechnology over drum pulping. It also shows the ink breakup with time inthe high consistency unit (drum pulping run 30 minutes with only a 30minute sample tested) and the initial high rate of ink breakup in theinitial 10 to 15 minutes of pulping.

FIGS. 4A and 4B shows the ERIC and brightness levels of the pulps andthe clear relationship between ERIC (fine ink) and loss of brightness.These experiments confirmed that low energy drum pulping insufficientlybreaks up ink particles resulting in a “dirtier” pulp produced. It isalso clear that high consistency pulping can be run too long so that theink released from the fibers is excessively broken down into ERIC whichattaches to the fibers resulting in a permanent brightness loss.

FIG. 5 shows the visible ink profile of drum stock and High Consistencyor Hi Con stock when deinked and bleached (ink specks larger than 225 μmin ppm). Unexpectedly the lower ink level of high consistency pulpingpersisted through the entire deink process. The high consistency pulphad a visible ink level 46% lower than the drum pulped wastepaper. Thisgoes against conventional belief that the mechanical action of kneadingand disk dispersion in subsequent processing would break up the visibleinks remaining after drum pulping.

FIG. 6 shows the brightness profile of the same pulps as FIG. 5. FIG. 6shows the reduced brightness coming out of the high consistency pulperpersisted throughout the deinking and bleaching process. The conclusionbeing the ink was smeared or reattached onto the fibers after about12-15 minutes of pulping. This was a very unexpected result andsignificant for both deinking design and for pulping technologyselection.

Further preliminary work included an analysis of deinking systemperformance versus pulping technology. It confirmed that highconsistency pulping delivers a lower finished pulp visible ink levelregardless of the deinking system technology. The work also showed highconsistency pulping was often “over pulping” resulting in permanentbrightness loss. With respect to drum pulping, it was found that drumpulping under-pulps, resulting in high ink and low yield.

Further work discovered more unexpected results. First, the power drawof the pulper motor, in terms of percent full load amps, showedsignificant batch to batch variability with no obvious cause. The actualpower output of the motor, in kW (as measured by the variable speeddrive), was added to the logged data (the “Pi” system) to assess causesof the variability. After an analysis of the Pi data additional trialwork was done to develop the relationships between the pulp consistency,the rotor speed and the power application. The expectation was that thepower demand would drop with pulper consistency. This is based on theprinciple that pulp viscosity, especially at higher consistency, willdrop as more dilution water is added; lower viscosity was expected tofacilitate motion of the pulp slurry around the pulper. However, it wasseen that in many of the trials the power load on the motor increased asdilution water was added. This was totally unexpected. It is postulatedthat the pulp viscosity may have been so high that the pulp flow intothe rotor became insufficient to fully load the motor; the rotor wasbeing starved of pulp and flow was short circuiting in the system.

This short circuiting of the flow is not easily discernable from thepower draw of the rotor motor—although it may be identified after thecycle is complete. In many cases there was an observed increase in powerdraw right after the post pulping dilution water (“topping water”) isadded. This power increase is a consequence of the dilution waterreducing the viscosity sufficiently for the pulp to start moving andmore fully engage the rotor. In some initial trials, also observed wasthat the visible ink level was dropping significantly after dilutionwater addition. This is explained by the mixing of the under pulpedpaper with the lower ink level over pulped paper after dilution. Notethat the pulp can only be sampled from the top of the pulper, so onlythe under pulped fiber can be tested if the circulation stops. Forexample, FIG. 7 shows typical pulper power cycles on a commercial batchpulper. Many of the pulpers show both the flat power curve and theincrease in power at the end of the cycle after addition of toppingwater. Two of the curves show a steadily increasing motor load followedby a reduction in power after the water addition. It is believed thesetwo pulpers had adequate poloidal flow throughout the cycle. It shouldbe noted that it would be very difficult to make real time adjustmentsto the viscosity (via water addition) in the very rapid 12 minute cyclebased on the power curve data alone.

Ultrasonic testing verified the hypothesis that loss of flow in thepulpers is very common and significantly impacts the quality of the pulpproduced. An analysis of the pulper operating systems, for example, thecalculated water addition based on the wastepaper charge, showed therewas very little variability in water addition, but high variability inproduct quality. This finding confirmed that variability was due inlarge part to the variability of the wastepaper itself.

It was determined that that high consistency pulping, utilizing a highenergy, short cycle pulping method could produce a high yield, low inklevel and high brightness pulp to feed into de-inking systems, whilegreatly reducing energy costs for recycling. Running closer to fullmotor load through consistency increases and/or higher loadings has thefollowing benefits:

-   -   10-30% energy savings (running fewer pulpers)    -   Improved deinking    -   Reduction in pulping time        Reducing pulping time to ˜10 minutes has the following benefits:    -   40-50% power savings    -   Potential higher finished stock brightness

Another key feature is to use a variable power rotor in the pulper toallow the energy level to be controlled. A particularly advantageousoperating mode is to use a variable speed pulper drive and reduce powerafter 10 minutes and during charge/idle times. This protocol has thefollowing benefits:

-   -   30-60% power savings    -   Higher finished stock brightness    -   Easier logistics—can test pulpers @ 10 minutes while rotor is in        idle mode    -   Mitigates downside risk of reduced pulper time (can add power,        if necessary)    -   Reduced maintenance (less torque stress on rotor and gearboxes)    -   Improved dumping and detrashing cycle times

The rotor speed will impact the time it takes to empty the pulper duringfiber extraction and during the dump cycle—adjusting the speed of therotor allows for faster turnaround times between batches of pulp.Typically, rotor speed is reduced well below speeds that circulate theslurry upwardly in the tub.

Another operating feature of the invention is to dilute the charge after10 minutes or after a particular energy input to preserve brightness(keep ERIC low). This aspect provides the following benefits:

-   -   20-30% power reduction    -   Higher finished stock brightness    -   Lower ink count    -   2-5% Increased Yield

Additional Trials

Three pulper runs were tested at a pulping consistency of 15-18%. Thewastepaper feed was higher brightness and quality intended forproduction of clean bright pulps. The pulpers were sampled at 2 minuteintervals with the start time at the ramp up of the pulper rotor to fullspeed. Normally the total pulping time is set to 12 minutes but thesepulpers were extended to develop a full Data Set. After 20 minutes apower curve was developed by adjusting the pulper speed to 800, 900,1000, 1100, and 1200 rpms and noting the motor power draw. Approximately1000 gallons of dilution water was then added to the pulper and thepower curve repeated. Another 1000 gallons of dilution water was addedand a 3rd power curve generated. Finally, the pulper was allowed tocontinue normally, dump and the next cycle started. For each set thetesting after the power curves showed a significant drop in dirt and anincrease in VSI. This was unexpected because the power addition was verylow in the short time it took to run the power curves.

The actual power draw for each of these was plotted with time. Set 1showed a different power curve compared to sets 2 and 3. The reason forthe power curve difference was not known at the time but has beenattributed to the total loss of poloidal flow in the pulper in sets 2and 3 versus a partial loss in Set 1. Results appear in FIGS. 8 and 9.

The loss of flow also explains the strange power curves for thesepulpers. In FIG. 10 there is plotted the power curve change relative tothe undiluted pulp for each set and dilution. Set 1 shows the powergenerally dropping with dilution—although less of a drop than expected.Sets 2 & 3 show the power significantly increasing with each dilution—anunexpected result.

In another series of tests on commercial units, initially all pulperswere randomly tested for flow velocity at several points in the pulpertub. This data showed clearly the regular loss of significant flow inthe tub after 3-5 minutes of pulping time. The testing also showed theshort circuiting in the pulper where pulp at the bottom of the tub wascycling through the rotor. The next tests were done to monitor the flowat 3 points in the tub for several pulping cycles. The points were“bottom” which was approximately 12 inches above start of the verticalsection of the pulper tub, “middle” which was 18 inches higher and “top”which was an additional 18 inches higher and roughly 1 to 2 feet belowthe top level of the pulp mass. Each point was tested every 90 seconds(a 30 second rotation between points) and the flows recorded. The startof the pulping, where the rotor motor was increased to 1200 rpm aftercharging, is the “0 Minutes” point for the following curves. FIG. 11shows the pulper energy level for the three runs. The three sets show asignificantly different energy curve during the pulping. Set 6 shows analmost flat energy curve throughout the pulping while Sets 4 & 5 showincreasing energy with time (with two different slopes).

The velocity data is plotted along with the power in FIG. 12 for Set 6.This set shows a flat energy curve and from the velocity data we clearlysee the loss of flow in the middle and top test points after about 4minutes. We also see some movement in the bottom which indicates shortcircuiting and over pulping of the fiber in the bottom of the pulper.

The velocity data is shown in FIG. 13 for Set 4. This set shows someflow throughout the cycle with the middle falling below about 0.1 ft/secoften during the cycle. This set also shows an increasing energy draw onthe rotor which is interpreted as consistent pulping throughout thecycle. Also note the small increase in energy applied compared to Set 6.

FIG. 2, for Set 5, shows the best pulping achieved of the three tested.This set had a steadily increasing energy application with sufficientflow in all regions of the pulper throughout the cycle. Set 5 isconsidered to be optimally executed.

A typical operating sequence for the pulper is as follows. Thewastepaper blend, initial water charge, charging sequence and otherparameters are held constant. Beginning with an empty pulper, thefollowing steps are carried out:

-   -   1. The rotor motor speed is set to a constant 360 rpm (36 rpm        rotor speed with a 10:1 gear ratio);    -   2. The weighing conveyor is charged with a target 25,000 lb. of        wastepaper;    -   3. A calculation is done based on the actual wastepaper weight        to determine the amount of water necessary to get to a 21.5 wt %        calculated consistency and 85% of this volume added to the        pulper;    -   4. The pulper motor is set to 1200 rpm, rotor speed 120 rpm;    -   5. The conveyor is started and wastepaper added to the pulper        over about a 1 to 2 minute timeframe;    -   6. The remaining 15% of the calculated water charge is added to        the pulper;    -   7. The pulper motor is placed into power mode and allowed to        adjust the speed to get to a target 900 kw, at the same time an        integrator is started to accumulate the total kWHr/ton;    -   8. The pulping cycled continue until the target kWHr/ton is        achieved;    -   9. The post pulping dilution water was added to the pulper to        achieve a calculated 12 wt % consistency and the pulper run in        power control mode for 3 minutes to mix;    -   10. The pulper is switched out of power mode into speed mode,        running the motor at 1000 rpm or so and the charge is was        removed and the pulper emptied and trash removed in preparation        for the next run.

In still yet further trials, a series of three (3) additional runs, DataSets 7, 8 and 9, were run on a pulper having the general configurationof FIG. 1 using the procedure generally indicated immediately above.Poloidal velocity was measured using an upper Doppler sensor. For thesethree runs an additional step was implemented at step 7 for purposes oftesting and characterizing the pulping operation. Under rotor dilutionwater was added at a controlled rate during the pulping cycle to developcombined power, speed and velocity control. For run #7 the addition ratewas set to 200 gallons per minute for the first 11 minutes of thepulping cycle. During the run the control system recorded all controldata at 6 second intervals indicated by the numerals on the abscissa ofFIG. 14.

The wt. fr. (wt % or consistency÷100) line shows the steady reduction incalculated consistency as the under rotor water is added. It also showsthe inflection point at the end of the pulping cycle where post pulpingdilution water is added.

The kW and RPM lines show the motor at 1200 rpm at the start of thecycle and the going to 1400 until the power reaches the target 900 kw.We then see the rotor speed decrease as the power is held constant. ThekW/RPM line combines these two and shows the applied power perrevolution increase throughout the pulping cycle until it reaches amaximum around about 0.77 kW/RPM.

The velocity curve shows the speed of the pulp as it rises up the pulperwall (based on the ultrasonic doppler meter). This curve is surprisingand unexpected in several ways. First, there appears to be norelationship between the rotor speed and the wall velocity when onewould expect a higher rotor speed to result in a higher rising speed.Next, the poloidal velocity curve shows a linear reduction from thebeginning to about period 75 (6 second intervals) and then a period ofinstability followed by a rising curve. The steady reduction at thebeginning is somewhat expected as the pulp mass is thickening as thefibers are individualized and starches and other additives are brokendown. The sharp increase in velocity at the end of the cycle (after thepost dilution water addition) is also expected as the pulp viscosity isdramatically reduced from the water addition. However, the period ofinstability followed by an upward trend (periods 75-100) is unexpected.Data Sets 8, 9 were generated using the same procedure and showed asurprisingly similar curve.

Data Set 8 results are shown on FIG. 15. While set up and run the sameas Data Set 7 the RPM and power lines show this pulper did not get totarget power (900 kw) until around period 100. This is an example of thewastepaper variability discussed above, although the same wastepapergrade was used as in Data Set 7, the same amount of water and theloading was used, it appears a different viscosity was exhibited. As inData Set 7, we see a steadily decreasing velocity until we hit a pointof instability around period 130. We see the same instability and then asteadily rising velocity until the large increase after the addition ofpost dilution.

In Data Set 9, FIG. 16, we see the similar velocity curve withinstability beginning around period 140 and then rising velocity afterperiod 150. Note, also, the wt. fr. curve inflection after interval 62.Prior to interval 62 the under rotor water flow was below target and thetarget flow of 200 gal/min was hit after interval 62. After this we seethe motor kw drop off for a number of periods then hit target afterperiod 100. It appears the dilution water was short circuiting somewhatafter the flow was established and staying around the rotor and then itthen mixed into the pulp uniformly. Despite the non-uniform wateraddition, there is shown a similar velocity curve as with Data Sets, 7,8. It is also seen that the kw/rpm values max out around 0.78 and thenhold steady no matter the dilution water added.

FIG. 17 presents velocity data for all three Data Sets 7, 8, 9. They arevery similar despite some time shifting due to differing pulpingprogress over time, most likely due to wastepaper variability, since allother features of the process are essentially the same.

FIG. 18 shows the velocity curves along with the calculated wt % datafor Data Sets 7, 8, 9. Vertical lines project the inflection points inthe velocity curves to consistency to show that while the curves havesimilar shapes, there is actually a different inflection point for eachvelocity curve. This indicates that the consistency is not the driverhere but rather the viscosity. That is, at a given wt % the viscosity ofa wastepaper mixture will vary due to variation in the wastepaper fibermix and additives; even though the wastepaper is nominally all the same“grade” of wastepaper.

In some cases it is convenient to use an under rotor addition point forwater, since a typical high consistency pulper includes a suitably sizedgallons/minute inlet at this location (up to 300 gal/minute).

Optimal pulping may be just before the unstable velocity point andinflection where additional water causes the velocity to increase. Onepreferred control scheme is a feedback control scheme with three targetvalues: motor power+rpm+velocity. Water addition is used to approachtarget values. One set of parameters during pulping using this approach(for a typical wastepaper charge) is around 900 kw motor power, +1200rpm motor speed (120 rpm rotor speed)+0.2 ft/sec poloidal velocity.

Another preferred control scheme is to use two target values: kw/motorrpm above 0.7 (or above 7 kw/rotor rpm) and velocity=0.2 ft/sec.

Still yet another option is to pulp to just after the poloidal velocityinstability point, i.e. where additional water shows an increasedvelocity. In practical terms this might be an easier place to controlpulping with an appropriate algorithm.

The foregoing protocols are based, in part, on the unexpected resultsseen with the invention. Pulping characteristics are perhaps morereadily observed by plotting poloidal pulp slurry velocity versus motorRPM for Data Set 7, as is seen in FIG. 19. During the first 8 minutes orso of pulping, poloidal slurry flow decreases linearly from the start ofwastepaper pulping to about 8 minutes as the motor speed decreases. Therelationship between motor RPM and poloidal velocity suggests thatrelatively more energy is being applied to pulping the wastepaper thanmaintaining or increasing velocity of the slurry. At about 8 minutes apoloidal slurry flow instability occurs over a period of 2 minutes or soas poloidal slurry velocity fluctuates before stabilizing again andincreasing linearly as motor speed decreases slightly and water isadded, indicating that relatively more energy is now being applied toincrease the velocity of the slurry rather than being applied to pulpthe wastepaper.

High consistency batch pulping starts out as a mixture of wastepaperpieces and water. The viscosity is nearly that of water and the flow isvery high. As the paper breaks down into smaller pieces+individualfibers+broken down ash and additives the viscosity increases. The higherviscosity puts more drag forces on the rotor which increases the powerrequired to spin the rotor. The higher viscosity also slows the flow ofpulp into the rotor due to gravity which can starve the rotor decreasingthe drag on the rotor and thus reducing power. So in the area where thevelocity is falling the viscosity increase from the breakdown of thewastepaper is higher than balanced by dilution water addition reducesviscosity. Eventually, a point is reached where the breakdown slows (thepulping is nearly finished). Now additional water results in a decreasein viscosity and the velocity increases. The power to rpm ratio, kw/rpm,is lower at the beginning of pulping and then increases as both theviscosity and engagement of the rotor increase. Around the inflectionpoint in velocity the kw/rpm ratio starts to level out and reaches amaximum as more water is added.

The velocity instability observed may be due to a change in the natureof the flow the pulp. It may be that the pulp around the rotor isactually becoming fluidized. The partially fluidizes pulp causes flowvariation as seen in the velocity meter bouncing. When a point isreached where there is a low enough viscosity that the pulp becomes amore stable fluid, the velocity starts rising. This is also the pointwhere the power addition of the rotor stabilizes (as indicated by a flatkw/rpm).

Pulping is likely most efficient just before the velocity instabilitypoint. When the pulp becomes fluidized (if indeed it is) the specificenergy input to the fiber decreases—less fiber “work” and more of theenergy is applied to pushing the pulp around. The velocity changessupport this theory. Prior to the transition point the velocity isdropping in the face of increased power addition so the power is goinginto the fiber and not into poloidal flow. After the transition theenergy increasingly goes into moving the pulp mass faster to less energyis going into the individual fibers.

Following in Table 1 are typical and preferred parameters for operatinga high consistency batch pulper of the class described below inconnection with FIG. 1.

TABLE 1 High Consistency Batch Pulper Operating Conditions QuantityTypical Range Preferred Range Charge Consistency (%) 10-30  12-25 EnergyInput (kwh/MT) 5-20   9-11.5 Pulping Duration (min) 6-25 7.5-20 Poloidal Velocity (ft/sec) 0.05-0.6  0.1-0.4 Rotor Speed (RPM) 60-180 75-140 Power Input (kw) 700-1500  800-1200 Power/rotor RPM Max (kw/RPM)6-10 7-9

Preferred Operation

The present invention is suitably implemented using an apparatus shownschematically in FIG. 1. In FIG. 1 there is shown a high consistencybatch pulper 10 which includes a tub 12 with a sidewall 15 provided withinternal baffle plates 14, 16, as well as a trash dump valve 18 and arotor 20. Rotor 20 is connected to a motor 22 provided with a variablepower drive indicated generally at 22A by way of a gearbox indicated at24.

Also provided are a plurality of Doppler velocimetry sensors 26, 28, 30located respectively near the bottom, middle and top of sidewall 15 oftub 12. In particular, sensors 26, 28, 30 may be located respectively atfrom 10-20% of a height, H, at from 40-60% of a height, H, and from70-90% of a height, H, wherein H is the height above a bottom 35 of tub12 and a normal operating level 33 indicated on FIG. 1. The sensors 26,28, 30 may be circumferentially staggered, if so desired, in order todetect flow at different portions of tub 12; that is, at differentpositions around the circumference. Poloidal flow is thus convenientlycharacterized by upward velocity of the slurry near the wall of thepulper in the direction indicated by arrow 34 using one or moredirectional Doppler effect ultrasonic flow meters which measure pulpslurry velocity in the direction indicated.

In operation wastepaper and water are charged through the upper portion32 of tub 12 to a normal operating level 33 and the pulper is operatedat a pulp consistency of anywhere from about 10% to 30% while rotor 20applies shear to the pulp slurry, causing the pulp slurry to circulatepoloidally at a poloidal velocity indicated at 34 of anywhere from 0.1ft/sec or above, up to 1 ft/sec or so. Rotor 20 is operated near maximumpower and poloidal velocity 34 is monitored. If poloidal velocity 34falls below a threshold of about 0.1 ft/sec, dilution water is added tocut the viscosity of the pulp slurry and maintain poloidal velocity 34at 0.1 ft/sec or above. Adding dilution water to maintain poloidalvelocity 34 also keeps rotor 20 operating at or near full power tomaximize energy input to the pulp. Typically the energy input to thewastepaper is in the range of from about 5 to 40 KwHr/MT over a pulpingoperating cycle of 10 minutes or so. At the end of pulping dilutionwater is added and the pulp is pumped out through an extraction plate(not shown). Tub 12 is rinsed a few times and then a trash dump valve 18opens and all of the large trash is flushed out of the pulper—wires,wood, plastic bottles, etc. The trash goes to a trommel screen whereresidual fiber is washed off and recovered. The large trash then goes tolandfill or, in some instances, plastic and metal recovery.

There are numerous ways to achieve this control with the optimalconfiguration dependent on the pulping equipment, wastepaper type anddesired pulping outcome.

One exemplary method comprises the following steps:

-   -   1. The pulper is charged with water (based on the actual weight        of the wastepaper and the target consistency) and then the        wastepaper added while the rotor is turning at a moderate speed.    -   2. Once the wastepaper and initial water charge are entirely in        the pulper the rotor is ramped to maximum speed limited only by        the maximum power draw of the drive motor. The rotor speed is        moderated during the pulper cycle if necessary to maintain motor        power draw less than 100% of the maximum. Depending on the        system design and loading, the power may begin to drop at some        point as the speed is ramped up. It is preferable to utilize a        smart controller algorithm to adjust the maximum speed if there        is a drop in power indicative of poor circulation and/or rotor        cavitation.    -   3. An accumulator for specific energy applied is started at the        beginning of the pulping. The accumulator uses the actual pulp        weight and the measured kW output of the motor to calculate        specific energy.    -   4. The linear velocity at several points in the pulp mass is        monitored and additional aliquots of dilution water are added to        the pulper to maintain linear velocity above about 0.03 m/sec        (0.1 ft/sec) at all points in the rising pulp mass.    -   5. Once the target specific energy is reached the “topping”        dilution water is added and the rotor speed reduced to minimize        the energy application to the pulp and prevent over pulping and        the potential brightness loss.    -   6. The pulp is extracted from the pulper tub. The rotor speed is        adjusted to give the shortest extraction time.    -   7. The trash is removed (unpulped wastepaper and nonfiber        contaminants too large to get through the extraction plate) and        the system prepared to start the next cycle.

Another exemplary method comprises the following steps:

-   -   1. The pulper is charged with about 40% to 60% of the total        water required (based on the actual weight of the wastepaper and        the target consistency) and then the wastepaper added while the        rotor is turning at a moderate speed.    -   2. Once the wastepaper and initial water charge are entirely in        the pulper the rotor is ramped to maximum speed limited only by        the maximum power draw of the drive motor. The rotor speed is        moderated during the pulper cycle if necessary to maintain motor        power draw less than 100% of the maximum. The maximum speed may        also be limited if there is excessive splashing in the pulper at        the lower level. This is indicated by either an excessive        velocity reading by the flow meter or as an excessive        fluctuation in the flow reading.    -   3. An accumulator for specific energy applied is started at the        start of the pulping. The accumulator uses the actual pulp        weight and the measured kW output of the motor to calculate        specific energy.    -   4. The linear velocity at several points in the pulp mass is        monitored and additional dilution water is added to the pulper        to maintain linear velocity above about 0.03 m/sec (0.1 ft/sec)        at all points in the rising pulp mass. Water addition may be        continuous or in aliquots.    -   5. Once the target specific energy is reached the “topping”        dilution water is added and the rotor speed reduced to minimize        the energy application to the pulp and prevent over pulping and        the potential brightness loss.    -   6. The pulp is extracted from the pulper tub and the system        prepared to start the next cycle.

Still yet another exemplary method involves a two stage pulping. Theinitial pulping is at high consistency and the second stage is at alower consistency. The lower consistency stage allows a higher totalenergy input with less risk of brightness loss. This protocol willincrease the yield and reduce the dirt level:

-   -   1. The pulper is charged with about 40% to 60% of the total        water required (based on the actual weight of the wastepaper and        the target consistency) and then the wastepaper added while the        rotor is turning at a moderate speed.    -   2. Once the wastepaper and initial water charge are entirely in        the pulper the rotor is ramped to maximum speed limited only by        the maximum power draw of the drive motor. The rotor speed is        moderated during the pulper cycle if necessary to maintain motor        power draw less than 100% of the maximum.    -   3. An accumulator for specific energy applied is started at the        start of the pulping. The accumulator uses the actual pulp        weight and the measured kW output of the motor to calculate        specific energy.    -   4. The linear velocity at several points in the pulp mass is        monitored and additional dilution water is added to the pulper        to maintain linear velocity above about 0.03 m/sec (0.1 ft/sec)        at all points in the rising pulp mass. Water addition may be        continuous or in aliquots.    -   5. Once an initial target specific energy has been reached        dilution water is added to decrease the consistency of the batch        below about 10% so that ink deposition due to additional pulping        is reduced or eliminated altogether.    -   6. The pulping continues until the second specific energy target        has been reached then the topping dilution water is added and        the rotor speed reduced to minimize the energy application to        the pulp and prevent over pulping and the potential brightness        loss.    -   7. The pulp is extracted from the pulper tub and the system        prepared to start the next cycle.

Multi-stage pulping can be carried out in a single batch pulpersequentially as indicated above or using continuous pulpers or batchpulpers arranged in series, if so desired. Multi-stage pulping isindicated schematically in FIG. 20. In a first stage 100 pulping iscarried out at high consistency, in the range of 10-30% solids with flowcontrol, as indicated herein, to avoid over-pulping. In a second stage200 pulping is continued at a mid-consistency range of 6-8% solids or atlow consistency (<6%) where permanent brightness loss due toover-pulping does not typically occur.

A still further exemplary method involves re-charging the highconsistency pulper without removing the trash which can shorten cycletime by 50% or more. The velocity sensor allows one to skip the trashremoval step for 2 or more cycles. The pulper as in the first or secondmethod, pulping is performed to the desired endpoint, the topping wateris added and the pulp pumped out through the extraction plate. Thepulper is then recharged with more wastepaper and water and the velocitysensor makes it possible to adjust the charge to the correct viscosity.The charge is pulped to the predetermined energy input, topping water isadded and the recharge is then pumped out of the pulper. Depending onthe trash level, one might then either remove the trash or if low enoughthe process may be repeated with a second recharge and another anotherpulping cycle performed without detrashing the pulper. This exemplarymethod can be summarized as:

-   -   1. The pulper is charged with water (based on the actual weight        of the wastepaper and the target consistency) and then the        wastepaper added while the rotor is turning at a moderate speed.    -   2. Once the wastepaper and initial water charge are entirely in        the pulper the rotor is ramped to maximum speed limited only by        the maximum power draw of the drive motor. The rotor speed is        moderated during the pulper cycle if necessary to maintain motor        power draw less than 100% of the maximum. Depending on the        system design and loading, the power may begin to drop at some        point as the speed is ramped up. It is preferable to utilize a        smart controller algorithm to adjust the maximum speed if there        is a drop in power indicative of poor circulation.    -   3. An accumulator for specific energy applied is started at the        beginning of the pulping. The accumulator uses the actual pulp        weight and the measured kW output of the motor to calculate        specific energy.    -   4. The linear velocity at several points in the pulp mass is        monitored and additional aliquots of dilution water are added to        the pulper to maintain linear velocity above about 0.03 m/sec        (0.1 ft/sec) at all points in the rising pulp mass.    -   5. Once the target specific energy is reached the “topping”        dilution water is added and the rotor speed reduced to minimize        the energy application to the pulp and prevent over pulping and        the potential brightness loss.    -   6. The pulp is extracted from the pulper tub.    -   7. Steps 1 through 6 are repeated until flow readings indicate        trash levels are excessive; whereupon the trash is removed        (unpulped wastepaper and nonfiber contaminants too large to get        through the extraction plate) and the system prepared to start        the next cycle.

Still other processing protocols may be employed depending on the feed,product quality requirements and so forth.

In addition to improvements in brightness and dirt removal, the presentinvention also enables higher yields on wastepaper re-pulping as wasfound in connection with pulping trials by measuring electrical powerdelta to a motor powering a rotating trommel screen during trash removalcycles of a high consistency batch pulper. The power delta correlateswith trash including the number of rejects (unpulped wastepaper) presenton the trommel screen. It was found that re-pulping wastepaper utilizingthe teachings of the present invention used 31% less applied energy tothe rotating trommel screen as compared with conventional recipe basedre-pulping of wastepaper. The power reduction to the trommel screen seenwith the invention indicates a decrease in trash which correlatesgenerally to a corresponding reduction in rejects. This represents adramatic and unexpected improvement in yield from the wastepaper.

Embodiments of the Invention

Various exemplary embodiments and features of the invention areenumerated below.

There is thus provided in accordance with the present inventionEmbodiment No. 1 which is a method of pulping wastepaper comprising:

-   -   (a) providing a pulping vessel with a rotor having a power drive        and at least one flow sensor adapted to measure slurry flow        within the pulping vessel;    -   (b) charging the pulping vessel with wastepaper and water to        form a slurry, the amounts of wastepaper and water being present        such that the slurry has consistency in the range of from 10% to        30%;    -   (c) pulping the wastepaper charge in the pulping vessel while        monitoring poloidal slurry flow in the pulping vessel; and    -   (d) from time-to-time, adding water to the pulping vessel when        the poloidal flow in the pulping vessel falls below a        predetermined lower threshold flow value in order to reduce        viscosity and thereby restore poloidal flow within the pulping        vessel.

Embodiment No. 2 is the method according to Embodiment No. 1, whereinthe pulping vessel is provided with an ultrasonic Doppler flow sensor.

Embodiment No. 3 is the method according to Embodiment No. 2, whereinthe pulping vessel is provided with a plurality of ultrasonic Dopplerflow sensors.

Embodiment No. 4 is the method according to Embodiment No. 3, whereinthe pulping vessel has a fill height, H, above a bottom of the pulpingvessel and a first ultrasonic Doppler flow sensor disposed at a heightof from 10-20% of height, H, a second ultrasonic Doppler flow sensordisposed at a height of from 40-60% of height, H, as well as a thirdultrasonic Doppler flow sensor disposed at a height of from 70-90% ofheight, H.

Embodiment No. 5 is the method according to any one of the foregoingEmbodiments, wherein the lower threshold flow value is 0.05 ft/sec orabove.

Embodiment No. 6 is the method according to Embodiment No. 5, whereinthe lower threshold flow value is 0.1 ft/sec or above.

Embodiment No. 7 is the method according to any one of Embodiment Nos.1-6, wherein the lower threshold flow value is from 0.05 ft/sec to 0.5ft/sec.

Embodiment No. 8 is the method according to any one of Embodiment Nos.1-7, wherein the slurry is pulped at a consistency of from 12% to 25%.

Embodiment No. 9 is the method according to any one of Embodiment Nos.1-8, wherein the power drive is a variable power drive operated at apre-selected power level.

Embodiment No. 10 is the method according to Embodiment No. 9, whereinthe variable power drive is a variable frequency power drive comprisingan AC drive and an AC motor.

Embodiment No. 11 is the method according to Embodiment No. 10, whereinthe AC motor is operated at a power level above 80% of the AC motor'smaximum power rating.

Embodiment No. 12 is the method according to Embodiment No. 11, whereinthe AC motor is operated at a power level above 90% of the AC motor'smaximum power rating.

Embodiment No. 13 is the method according to Embodiment No. 12, whereinthe AC motor is operated at a power level of up to 120% of the ACmotor's maximum power rating.

Embodiment No. 14 is the method according to any one of Embodiment Nos.1-13, wherein the slurry is pulped at a specific energy input of from 5KwHr/MT to 40 KwHr/MT.

Embodiment No. 15 is the method according to Embodiment No. 14, whereinthe slurry is pulped at a specific energy input of from 5 KwHr/MT to 30KwHr/MT.

Embodiment No. 16 is the method according to Embodiment No. 14, whereinthe slurry is pulped at a specific energy input of from 5 KwHr/MT to 25KwHr/MT.

Embodiment No. 17 is the method according to Embodiment No. 14, whereinthe slurry is pulped at a specific energy input of from 7.5 KwHr/MT to20 KwHr/MT.

Embodiment No. 18 is the method according to Embodiment No. 14, whereinthe slurry is pulped at a specific energy input of from 8 KwHr/MT to 15KwHr/MT.

Embodiment No. 19 is the method according to any one of Embodiment Nos.1-18, operated in a batch mode.

Embodiment No. 20 is the method according to Embodiment No. 19, whereinthe slurry is pulped from 5 minutes to 20 minutes.

Embodiment No. 21 is the method according to Embodiment No. 19, whereinthe slurry is pulped from 7.5 minutes to 15 minutes.

Embodiment No. 22 is the method according to any one of Embodiment Nos.19-21, further comprising recovering the pulped wastepaper and repeatingthe method of pulping wastepaper without removing residual trash fromthe pulping vessel.

Embodiment No. 23 is the method according to any one of Embodiment Nos.1-22, further comprising diluting the charge to the pulper with water toa consistency lower than 10% and pulping the diluted charge.

Embodiment No. 24 is the method according to Embodiment No. 23, whereinthe charge to the pulper is diluted to a consistency in the range offrom 6-8%.

Embodiment No. 25 is a method of making absorbent sheet comprising:

-   -   (a) obtaining papermaking fiber from wastepaper by way of        pulping wastepaper according to the method of any one of        Embodiment Nos. 1 to 22;    -   (b) recovering papermaking fiber from the pulping vessel,        optionally further processing the recovered fiber by de-inking        and/or bleaching the recovered fiber; and    -   (c) incorporating the recovered papermaking fiber into an        absorbent sheet.

Embodiment No. 26 is the method of making absorbent sheet accordingEmbodiment No. 25, wherein the sheet is a paper towel sheet.

Embodiment No. 27 is the method of making absorbent sheet according toEmbodiment No. 25, wherein the sheet is a paper tissue sheet.

Embodiment No. 28 is the method of making absorbent sheet according toany one of Embodiment Nos. 25-27, wherein the recovered papermakingfiber is incorporated into the absorbent sheet by way of a CWP Process.

Embodiment No. 29 is the method of making absorbent sheet according toany one of Embodiment Nos. 25-27, wherein the recovered papermakingfiber is incorporated into the absorbent sheet by way of a StructuredBasesheet Process.

Embodiment No. 30 is the method of making absorbent sheet according toany one of Embodiment Nos. 25-27, wherein the recovered papermakingfiber is incorporated into the absorbent sheet by way of a TAD Process.

Embodiment No. 31 is the method of making absorbent sheet according toany one of Embodiment Nos. 25-30, wherein the absorbent sheet comprisesat least 30% by weight of recovered papermaking fiber.

Embodiment No. 32 is the method of making absorbent sheet according toEmbodiment No. 31, wherein the absorbent sheet comprises at least 50% byweight of recovered papermaking fiber.

Embodiment No. 33 is the method of making absorbent sheet according toEmbodiment No. 31 or 32, wherein the absorbent sheet comprises from 30%to 80% by weight recovered papermaking fiber.

Embodiment No. 34 is a high consistency pulping apparatus comprising:

-   -   (a) a pulping tub for receiving a wastepaper slurry;    -   (b) a rotor disposed in the pulping tub, coupled to a power        drive; and    -   (c) at least one flow sensor attached to the pulping tub adapted        to measure slurry flow within the pulping vessel.

Embodiment No. 35 is the apparatus according to Embodiment No. 34,wherein the flow sensor is an ultrasonic Doppler flow sensor.

Embodiment No. 36 is the apparatus according to Embodiment No. 34 or 35,wherein the pulping tub is provided with a plurality of ultrasonicDoppler flow sensors.

Embodiment No. 37 is the apparatus according to Embodiment No. 36,wherein the pulping tub has a fill height, H, above a bottom of thepulping tube and a first ultrasonic Doppler flow sensor disposed at aheight of from 10-20% of height, H, a second ultrasonic Doppler flowsensor disposed at a height of from 40-60% of height, H, as well as athird ultrasonic Doppler flow sensor disposed at a height of from 70-90%of height, H.

Embodiment No. 38 is the apparatus according to any one of EmbodimentNos. 34-37, wherein the pulping tub has disposed therein one or morebaffle plates between the rotor and a sidewall of the pulping tub.

Embodiment No. 39 is the apparatus according to any one of EmbodimentNos. 34-38, wherein the power drive is a variable power drive.

Embodiment No. 40 is the apparatus according to Embodiment No. 39,wherein the variable power drive is a variable frequency power drivecomprising an AC drive and an AC motor.

Embodiment No. 41 is a method of pulping wastepaper comprising:

-   -   (a) providing a pulping vessel with a rotor having a power drive        and at least one flow sensor adapted to measure slurry flow        within the pulping vessel;    -   (b) charging the pulping vessel with wastepaper and water to        form a slurry, the amounts of wastepaper and water being present        such that the slurry has consistency in the range of from 10% to        30%;    -   (c) selecting target levels for (i) poloidal slurry flow in the        pulping vessel; (ii) power input to the power drive; and (iii)        rotor RPM;    -   (d) pulping the wastepaper charge in the pulping vessel while        monitoring (i) poloidal slurry flow in the pulping vessel; (ii)        power input to the power drive; and (iii) rotor RPM; and    -   (e) adding water to the pulping vessel during pulping of the        wastepaper charge to adjust measured levels of one or more        of (i) poloidal slurry flow, (ii) power input to the power        drive, and (iii) rotor RPM toward the corresponding target        value.

Embodiment No. 42 is the method according to Embodiment No. 41, whereinthe target level for poloidal slurry flow is in the range of from about0.1 ft/sec to about 0.4 ft/sec.

Embodiment No. 43 is the method according to Embodiment No. 42, whereinthe target level for poloidal slurry flow is from about 0.15 ft/sec toabout 0.25 ft/sec.

Embodiment No. 44 is the method according to any one of Embodiment Nos.41-43, wherein target level for power input to the power drive is in therange of from about 700 kw to about 1500 kw.

Embodiment No. 45 is the method according to Embodiment No. 44, whereintarget level for power input to the power drive is in the range of fromabout 800 kw to about 1200 kw.

Embodiment No. 46 is the method according to any one of Embodiment Nos.41-45, wherein the target level for rotor RPM is in the range of fromabout 60 to about 180.

Embodiment No. 47 is the method according to Embodiment No. 46, whereinthe target level for rotor RPM is in the range of from about 75 to about140.

Embodiment No. 48 is a method of pulping wastepaper comprising:

-   -   (a) providing a pulping vessel with a rotor having a power drive        and at least one flow sensor adapted to measure slurry flow        within the pulping vessel;    -   (b) charging the pulping vessel with wastepaper and water to        form a slurry, the amounts of wastepaper and water being present        such that the slurry has consistency in the range of from 10% to        30%;    -   (c) selecting target levels for the ratio of (i) power input to        the power drive/rotor RPM and (ii) poloidal slurry flow in the        pulping vessel;    -   (d) pulping the wastepaper charge in the pulping vessel while        monitoring (i) the ratio of power input to the power drive/rotor        RPM and (ii) poloidal slurry flow in the pulping vessel; and    -   (e) adding water to the pulping vessel during pulping of the        wastepaper charge to adjust measured levels of one or more        of (i) the ratio of power input to the power drive/rotor RPM        and (ii) poloidal slurry flow toward the corresponding target        values.

Embodiment No. 49 is the method according to Embodiment No. 48, whereinthe target level of ratio of power input to the power drive/rotor RMP isin the range of from 6 kw/RPM to 10 kw/RPM.

Embodiment No. 50 is the method according to Embodiment No. 49, whereinthe target level of ratio of power input to the power drive/rotor RMP isin the range of from 7 kw/RPM to 9 kw/RPM.

Embodiment No. 51 is the method according to any one of Embodiment Nos.48-50, wherein the target level for poloidal slurry flow is in the rangeof from about 0.1 ft/sec to about 0.4 ft/sec.

Embodiment No. 52 is the method according to Embodiment No. 51, whereinthe target level for poloidal slurry flow is in the range of from about0.15 ft/sec to about 0.25 ft/sec.

Embodiment No. 53 is a method of pulping wastepaper comprising:

-   -   (a) providing a pulping vessel with a rotor having a power drive        and at least one flow sensor adapted to measure slurry flow        within the pulping vessel;    -   (b) charging the pulping vessel with wastepaper and water to        form a slurry, the amounts of wastepaper and water being present        such that the slurry has consistency in the range of from 10% to        30%;    -   (c) selecting a target level for poloidal slurry flow within the        pulping vessel;    -   (d) pulping the wastepaper charge in the pulping vessel while        monitoring poloidal slurry flow in the pulping vessel; and    -   (e) adding water to the pulping vessel to adjust measured levels        of poloidal slurry flow toward the target value.

Embodiment No. 54 is the method according to Embodiment No. 53, whereinthe target level for poloidal slurry flow is in the range of from about0.1 ft/sec to about 0.4 ft/sec.

Embodiment No. 55 is the method according to Embodiment No. 54, whereinthe target level for poloidal slurry flow is in the range of from about0.15 ft/sec to about 0.25 ft/sec.

Embodiment No. 56 is a method of pulping wastepaper comprising:

-   -   (a) providing a pulping vessel with a rotor having a power drive        and at least one flow sensor adapted to measure slurry flow        within the pulping vessel;    -   (b) charging the pulping vessel with wastepaper and water to        form a slurry, the amounts of wastepaper and water being present        such that the slurry has consistency in the range of from 10% to        30%;    -   (c) pulping the wastepaper charge in the pulping vessel while        monitoring poloidal slurry flow in the pulping vessel;    -   (d) discontinuing pulping of the wastepaper charge after the        onset of instability of poloidal slurry flow.

Embodiment No. 57 is the method according to Embodiment No. 56, whereinpulping is discontinued within 5 minutes after the onset of instabilityof poloidal slurry flow.

Embodiment No. 58 is the method according to Embodiment No. 57, whereinpulping is discontinued within 2 minutes after the onset of instabilityof poloidal slurry flow.

Embodiment No. 59 is the method according to Embodiment No. 58, whereinpulping is discontinued within 1 minute after the onset of instabilityof poloidal slurry flow.

Embodiment No. 60 is a method of pulping wastepaper comprising:

-   -   (a) providing a pulping vessel with a rotor having a power drive        and at least one flow sensor adapted to measure slurry flow        within the pulping vessel;    -   (b) charging the pulping vessel with wastepaper and water to        form a slurry, the amounts of wastepaper and water being present        such that the slurry has consistency in the range of from 10% to        30%;    -   (c) pulping the wastepaper charge in the pulping vessel while        monitoring poloidal slurry flow in the pulping vessel;    -   (e) operating the pulping vessel including optionally adding        additional water to the pulping vessel in order to maintain        poloidal slurry flow within the range of from about 0.1 ft/sec        to about 0.4 ft/sec during pulping.

Embodiment No. 61 is the method according to Embodiment No. 60, whereinpoloidal slurry flow is maintained within the range of from about 0.15ft/sec to about 0.25 ft/sec during pulping.

The following levels and features may be used in connection with anypulping procedure of the present invention as target levels or operatinglevels maintained during pulping.

Consistency of the wastepaper charge is in the range of from about 10%to 30% consistency, suitably from 12% to 25% consistency.

Energy input to the wastepaper charge during pulping is in the range offrom about 5 kwh/MT to about 40 kwh/MT, suitably from 5 kwh/MT to 30kwh/MT or from 5 kwh/MT to 25 kwh/MT or from 7.5 kwh/MT to 20 kwh/MT. MTrefers to metric tons of dry wastepaper provided to the charge to thepulper; kwh to kilowatt hours of energy input provided to the pulperduring pulping of the wastepaper charge.

The lower threshold slurry flow value is in the range of 0.05 ft/sec to0.5 ft/sec, typically 0.05 ft/sec or 0.1 ft/sec.

Poloidal slurry flow is typically in the range of from 0.1 ft/sec to 0.4ft/sec during pulping; preferably in the range of from 0.15 ft/sec to0.25 ft/sec during pulping.

Power input to the rotor power drive is generally in the range of from700 to 1500 kw; preferably in the range of from 800 kw to 1200 kw.

Rotor speed revolutions per minute (RPM) is in the range of from 60 RPMto 180 RPM; typically within the range of from 75 RPM to 140 RPM.

The ratio of power input to the rotor RPM is in the range of from 6kw/RPM to 10 kw/RPM; suitably from 7 kw/RPM to 9 kw/RPM.

Pulping duration is generally from 5 minutes to 20 minutes; moretypically from 7.5 minutes to 15 minutes.

Pulping may be terminated after the onset of instability in poloidalslurry flow rather than after a target time interval. When the onset ofinstability is used as a marker for discontinuing pulping of awastepaper, pulping may be terminated within 5 minutes after the onsetof instability if poloidal slurry flow, within 2 minutes after the onsetof instability of poloidal slurry flow or at shorter or longer timesafter the onset of instability of poloidal slurry flow, depending onconditions.

While the invention has been described in detail, modifications withinthe spirit and scope of the invention will be readily apparent to thoseof skill in the art. Such modifications are also to be considered aspart of the present invention. In view of the foregoing discussion,relevant knowledge in the art and references discussed above inconnection with the foregoing description including the DetailedDescription and Background of the Invention, the disclosures of whichare all incorporated herein by reference, further description is deemedunnecessary. In addition, it should be understood from the foregoingdiscussion that aspects of the invention and portions of variousembodiments may be combined or interchanged either in whole or in part.Furthermore, those of ordinary skill in the art will appreciate that theforegoing description is by way of example only, and is not intended tolimit the invention.

What is claimed is:
 1. A method of pulping wastepaper comprising: (a)providing a pulping vessel with a rotor having a power drive and atleast one flow sensor adapted to measure slurry flow within the pulpingvessel; (b) charging the pulping vessel with wastepaper and water toform a wastepaper slurry, the amounts of wastepaper and water beingpresent such that the slurry has consistency in the range of from 10% to30%; (c) pulping the wastepaper slurry in the pulping vessel whilemonitoring poloidal slurry flow in the pulping vessel; and (d)maintaining poloidal slurry flow in the pulper at or above a lowerthreshold slurry flow value during pulping.
 2. The method according toclaim 1, wherein the pulping vessel is provided with an ultrasonicDoppler flow sensor.
 3. The method according to claim 1, wherein thepoloidal slurry flow is maintained at or above the lower thresholdslurry flow value for at least 75% of pulping duration.
 4. The methodaccording to claim 3, wherein poloidal slurry flow is maintained at orabove the lower threshold slurry flow value for at least 75% of pulpingduration by (i) adding additional water, (ii) adjusting power input tothe wastepaper slurry, (iii) adjusting rotor speed, or a combination ofone or more of (i), (ii) and (iii).
 5. The method according to claim 1,wherein the lower threshold slurry flow value is 0.05 ft/sec.
 6. Themethod according to claim 5, wherein the lower threshold slurry flowvalue is 0.1 ft/sec.
 7. The method according to claim 1, furthercomprising selecting a target level of energy input to the wastepapercharge and continuing pulping of the wastepaper charge until the targetlevel of energy input to the wastepaper charge is achieved.
 8. Themethod according to claim 6, wherein the target level of energy input tothe wastepaper charge is within the range of from 5 kwh/MT to 20 kwh/MT.9. The method according to claim 1, wherein the poloidal slurry flow inthe pulper is maintained within the range of from about 0.1 ft/sec toabout 0.4 ft/sec during pulping.
 10. The method according to claim 9,wherein the poloidal slurry flow in the pulper is maintained within therange of from about 0.15 ft/sec to about 0.25 ft/sec during pulping. 11.The method according to claim 1, wherein power input to the power driveis maintained within the range of from about 700 kw to about 1500 kwduring pulping.
 12. The method according to claim 1, wherein the rotorRPM is maintained within the range of from about 60 to about 180 duringpulping.
 13. The method according to claim 12, wherein the rotor RPM ismaintained within the range of from about of from about 75 to about 140during pulping.
 14. The method according to claim 1, wherein the ratioof power input to the power drive to rotor RPM is maintained within therange of from about 6 kw/RPM to about 10 kw/RPM during pulping.
 15. Themethod according to claim 1, wherein the method includes pulping thewastepaper charge in the pulping vessel while monitoring (i) poloidalslurry flow in the pulping vessel; (ii) power input to the power drive;and (iii) rotor RPM; and adding additional water to the pulping vesselduring pulping of the wastepaper charge to adjust measured levels of oneor more of (i) poloidal slurry flow, (ii) power input to the powerdrive, and (iii) rotor RPM toward a corresponding target level.
 16. Themethod according to claim 1, wherein the method includes pulping thewastepaper charge in the pulping vessel while monitoring (i) the ratioof power input to the power drive/rotor RPM and (ii) poloidal slurryflow in the pulping vessel; and adding additional water to the pulpingvessel during pulping of the wastepaper charge to adjust measured levelsof one or more of (i) the ratio of power input to the power drive/rotorRPM and (ii) poloidal slurry flow toward a corresponding target level.17. The method according to claim 1, wherein the method includes pulpingthe wastepaper charge in the pulping vessel while monitoring poloidalslurry flow in the pulping vessel; and discontinuing pulping of thewastepaper charge after an onset of instability in poloidal slurry flow.18. The method according to claim 17, wherein pulping is discontinuedwithin 2 minutes after the onset of instability of poloidal slurry flow.19. The method according to claim 1, wherein the method includes pulpingthe wastepaper charge in the pulping vessel while monitoring poloidalslurry flow in the pulping vessel; and adding additional water to thepulping vessel to adjust measured levels of poloidal slurry flow towarda target level.
 20. The method according to claim 1, wherein the methodincludes pulping the wastepaper charge in the pulping vessel whilemonitoring poloidal slurry flow in the pulping vessel; and operating thepulping vessel including adding additional water to the pulping vesselwhile maintaining poloidal slurry flow within the range of from about0.1 ft/sec to about 0.4 ft/sec.
 21. A method of making absorbent sheetcomprising: (a) obtaining papermaking fiber from wastepaper by way ofpulping wastepaper according to the method of claim 1; (b) recoveringpapermaking fiber from the pulping vessel, optionally further processingthe recovered fiber by de-inking and/or bleaching the recovered fiber;and (c) incorporating the recovered papermaking fiber into an absorbentsheet.
 22. A high consistency pulping apparatus comprising: (a) apulping tub for receiving a wastepaper slurry; (b) a rotor disposed inthe pulping tub, coupled to a power drive; and (c) at least one flowsensor attached to the pulping tub adapted to measure slurry flow withinthe pulping vessel.
 23. The apparatus according to claim 22, wherein theflow sensor is an ultrasonic Doppler flow sensor.
 24. The apparatusaccording to claim 23, wherein the pulping tub is provided with aplurality of ultrasonic Doppler flow sensors.
 25. The apparatusaccording to claim 22, wherein the power drive is a variable powerdrive.